C/EBPα:AP-1 leucine zipper heterodimers bind novel DNA elements, activate the PU.1 promoter and direct monocyte lineage commitment more potently than C/EBPα homodimers or AP-1

The basic-region leucine zipper (BR-LZ or bZIP) transcription factors dimerize via their LZ domains to position the adjacent BRs for DNA binding. Members of the C/EBP, AP-1 and CREB/ATF bZIP subfamilies form homodimeric or heterodimeric complexes with other members of the same subset and bind-specific DNA motifs. Here we demonstrate that C/EBPα also zippers with AP-1 proteins and that this interaction allows contact with novel DNA elements and induction of monocyte lineage commitment in myeloid progenitors. A leucine zipper swap:gel shift assay demonstrates that C/EBPα zippers with c-Jun, JunB or c-Fos, but not with c-Maf or MafB. To evaluate activities of specific homodimers or heterodimers we utilized LZs with acid (LZE) or basic (LZK) residues in their salt bridge positions. C/EBPαLZE:C/EBPαLZK preferentially binds a C/EBP site, c-JunLZE:c-FosLZK an AP-1 site and C/EBPαLZE:c-JunLZK a hybrid element identified as TTGCGTCAT by oligonucleotide selection. In murine myeloid progenitors, C/EBPα:c-Jun or C/EBPα:c-Fos LZE:LZK heterodimers induce monocyte lineage commitment with markedly increased potency compared with C/EBPα or c-Jun homodimers or c-Jun:c-Fos heterodimers, demonstrating a positive functional consequence of C/EBP:AP-1 bZIP subfamily interaction. C/EBPα:cJun binds and activates the endogenous PU.1 promoter, providing one mechanism for induction of monopoiesis by this complex.